1.编写一个 SQL 查询，查找所有至少连续出现三次的数字。
+----+-----+
| Id | Num |
+----+-----+
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 4 | 2 |
| 5 | 1 |
| 6 | 2 |
| 7 | 2 |
+----+-----+
例如，给定上面的 Logs 表， 1 是唯一连续出现至少三次的数字。
+-----------------+
| ConsecutiveNums |
+-----------------+
| 1 |
+-----------------+
select Num as ConsecutiveNums from Logs where Id in (
select distinct l1.Id from Logs l1,Logs l2,Logs l3
where (l1.Num = l2.Num and l2.Num = l3.Num and (
(l1.Id + 1= l2.Id and l2.Id +1 = l3.Id)or
(l3.Id + 1= l2.Id and l2.Id +1 = l1.Id)or
(l3.Id + 1= l1.Id and l1.Id +1 = l2.Id)
)
)
order by l1.Id) group by Num
2.编写一个 SQL 查询，查找 Person 表中所有重复的电子邮箱。
示例：
+----+---------+
| Id | Email |
+----+---------+
| 1 | a@b.com |
| 2 | c@d.com |
| 3 | a@b.com |
+----+---------+
根据以上输入，你的查询应返回以下结果：
+---------+
| Email |
+---------+
| a@b.com |
+---------+
select Email from Person group by Email having count(Email)>1
3.编写一个 SQL 查询，来删除 Person 表中所有重复的电子邮箱，重复的邮箱里只保留 Id 最小 的那个。
+----+------------------+
| Id | Email |
+----+------------------+
| 1 | john@example.com |
| 2 | bob@example.com |
| 3 | john@example.com |
+----+------------------+
Id 是这个表的主键
delete p1 from Person p1,Person p2 where p1.Email=p2.Email and p1.Id>p2.Id
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